Solar selective surface coatings known in the art usually comprise an outer layer of a material which absorbs strongly in the solar energy spectral range but which is transparent to infra-red radiation. In the interest of reducing thermal losses, the outer layer is deposited on an inner layer of a material which provides high reflectivity and, hence, low emissivity in the infra-red spectrum. A typical such selective surface coating is disclosed in U.S. Pat. No. 4,339,484, the surface coating comprising a metal-carbide solar energy absorptive outer layer and a copper infra-red reflective base coating. The metal-carbide absorptive layer is graded such that it has a high carbide-to-metal ratio near its outer surface and a high metal-to-carbide ratio adjacent the interface between the absorptive surface and the reflective base coating.
A problem that is inherent in the traditional selective surface coatings is that the emissivity of the total surface coating is much higher than the emissivity of the reflective base layer alone. One of the major reasons for this is that the emissivity of a material increases if the optical constants of a superimposed medium are different from those of free space, and this normally is the case with materials that are employed for absorptive surface coatings. Such materials, if semiconductors, may have refractive indices in the order of 2 to 10 times greater than that of free space and, therefore, the coupling of infra-red radiation across the metal-semiconductor interface may be significantly greater than that of a metal-air interface.